This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A371991 #21 Jun 19 2025 00:09:58
%S A371991 1,1,0,1,1,2,3,6,6,16,21,42,63,129,201,404,685,1340,2385,4625,8487,
%T A371991 16409,30735,59282,112530,217182,415605,803076,1545463,2990945,
%U A371991 5778267,11201472,21702645,42140890,81830744,159139428,309590883,602935713,1174779207,2290915478,4469734225,8726815041,17047041429,33319598126
%N A371991 Number of different closest packings of equal spheres for hexagonal crystals having repeat period n.
%H A371991 J. E. Iglesias, <a href="https://doi.org/10.1524/zkri.1981.155.1-2.121">A formula for the number of closest packings of equal spheres having a given repeat period</a>, Z. Krist. 155 (1981) 121-127, Table 1.
%F A371991 a(n) + A371992(n) = A000046(n).
%F A371991 a(n+1)/a(n) = 2 - 2/n + O(1/n^2). - _M. F. Hasler_, Jun 09 2025
%o A371991 (PARI) apply( {A371991(n)=A000046(n)-A371992(n)}, [1..40]) \\ _M. F. Hasler_, Jun 09 2025
%Y A371991 Cf. A000046, A371992.
%K A371991 nonn
%O A371991 1,6
%A A371991 _R. J. Mathar_, Apr 15 2024
%E A371991 Offset changed to 1 and a(1) = 1 prefixed by _M. F. Hasler_, Jun 09 2025